High-performance display devices, such as liquid crystal displays (LCDs) and plasma displays, are commonly used in various electronics, such as cell phones, laptops, electronic tablets, televisions, and computer monitors. Currently marketed display devices can employ one or more high-precision glass sheets, for example, as substrates for electronic circuit components, or as color filters, to name a few applications. The leading technology for making such high-quality glass substrates is the fusion draw process, developed by Corning Incorporated, and described, e.g., in U.S. Pat. Nos. 3,338,696 and 3,682,609, which are incorporated herein by reference in their entireties.
The fusion draw process can utilize a fusion draw machine (FDM) comprising a forming body (e.g., isopipe). The forming body can comprise an upper trough-shaped portion and a lower portion having a wedge-shaped cross-section with two major side surfaces (or forming surfaces) sloping downwardly to join at a root. During the glass forming process, the molten glass can be delivered to one end of the isopipe (“delivery end”) and can travel down the length of the isopipe while flowing over the trough side walls (or weirs) to an opposite end (“compression end”). The molten glass can flow down along the two forming surfaces as two glass ribbons, which ultimately converge at the root where they fuse together to form a unitary glass ribbon. The glass ribbon can thus have two pristine external surfaces that have not been exposed to the surface of the forming body. The ribbon can then be drawn down and cooled to form a glass sheet having a desired thickness and a pristine surface quality.
Isopipes used in the fusion draw process are often large bodies formed from heavy, refractory ceramic materials. Isopipes can be subjected to rigorous operating conditions, such as high temperatures, for extended periods of continuous use, e.g., up to several years or more. During operation, the refractory body may deform (e.g., sag) in the middle, which can ultimately change the molten glass flow characteristics in the FDM. Higher temperature operations can accelerate isopipe deformation such as sag due to creep of the refractory material. Isopipe sag can be partially mitigated by applying a horizontal compression force below the isopipe neutral axis, for instance, at the ends of the lower wedge-shaped portion. However, this compression force itself creates stress in the forming body, which can lead to static fatigue of the refractory material. Stress and sag should therefore be balanced and minimized to extend the life of the forming body and/or maintain glass quality.
Consumer demand for high-performance displays with ever growing size and image quality requirements drives the need for improved manufacturing processes for producing large, high-quality, high-precision glass sheets. Larger (e.g., longer and heavier) isopipes for producing large glass sheets can have increased likelihood of failure due to sag and/or stress over time. Accordingly, it would be advantageous to provide methods and apparatuses for mitigating isopipe sag and providing improved isopipe support. In various embodiments, the methods and apparatuses disclosed herein can minimize or prevent isopipe sag and/or stress, which can extend the useful life of the forming apparatus and/or maintain high glass quality over the lifetime of the apparatus.